Non-destructive testing of pipe has been done for many years utilizing ultrasonic transducers, eddy current measurements, x-ray and other techniques. Operators using pulse echo techniques with ultrasonic transducers can determine wall thickness, which is a measure of any corrosion that has occurred. For welds, operators have used flight diffraction (“TOFD”) techniques with ultrasonic transducers. Also, a method known as pulse echo shear wave has been combined with TOFD transducer measurements to inspect portions of the weld that are missed by the TOFD transducer.
One type of pipe that requires periodic inspection is a drilling riser. Drilling risers, which are utilized for offshore drilling, extend from the drilling rig to a blowout preventer and lower marine riser package, which connect to a subsea wellhead. Drilling risers are made up of sections bolted together with flanges, each section being typically from 60-90 feet in length. Each drilling riser section has a central riser pipe that is normally about 18-24 inches in diameter. Several auxiliary lines are mounted to the exterior of the central riser pipe, the auxiliary lines being used for a choke, kill and hydraulic boost purposes. The auxiliary lines are smaller in diameter and mounted parallel and offset to the axis of the central riser pipe. Normally there will be at least one weld within each riser section, this being a center weld that connects two tubular pipes together to form the riser section. Also, normally the flange connectors are mounted to the ends of the riser sections by welding. Many risers also have buoyant jackets mounted to the exterior.
A drilling vessel may have several thousand feet of riser pipe, depending on the depth to which it is rated. During use, drill pipe with drill bits on the end, casing, and other well tools are lowered through the riser. Drilling mud returns up the riser. The auxiliary lines are pressurized for various purposes from time to time. The drilling riser is re-used after each well. Consequently it is necessary to periodically inspect the riser to make sure that it has no weaknesses that could result in leakage.
Inspection in the past has been done primarily by transporting the riser sections to a facility on land that performs the inspection services. The facility removes the buoyancy jackets and auxiliary lines from each section. The riser sections are cleaned and inspected from the exterior with various ultrasonic transducers. If the riser is coated with an epoxy, it must be removed at each inspection site. After inspection, the riser sections are reassembled and shipped back to the drilling vessel.
The transport of the riser sections to a testing facility on land is expensive. Also, it is time consuming to transport, clean, disassemble, inspect and reassemble the riser sections. During this time, unless a spare drilling riser can be obtained, the drilling rig would not be able to operate. Drilling rigs are very costly on a daily basis.
It has been proposed to inspect the drilling risers at the drilling vessel. Many drilling vessels have the ability to stack the riser sections horizontally on the vessel while not in use. However, there are a number of problems in doing so. The interior of the drilling riser is often not very clean, and may be coated with dried drilling mud. The central riser pipe is often out of round in portions. The welded areas of the central pipe may be misaligned slightly. Also, there is normally not much access room on the drilling rig at the ends of each riser section for staging the equipment necessary to do the inspection.